When a Raman active medium is placed in an optical cavity, a large enhancement in the scattering efficiency can be realized. The enhancement can be observed when the cavity is made resonant with either the wavelength of the excitation light or the wavelength of the Raman scattered light. The effect is maximized when the cavity is made simultaneously resonant with both, i.e., when a double resonance is obtained. The exact magnitude of the enhancement depends on, among other factors, the finesse of the cavity at the two wavelengths and the volume of the mode sustained by the microcavity.
In the prior art the double resonance condition could be achieved only for a very thin solid sample with a thickness of less than one wavelength. The sample was made to have plane parallel surfaces, effectively creating a planar resonator with two flat partial reflectors forming the cavity. Because of the fixed spacing of the cavity, resonance with the wavelength of the excitation light could be established only for an off-axis laser beam. As a result, the resonance was accompanied by a high walk-off loss. Furthermore, planar resonators are inherently only quasi-stable, and are therefore precluded from having a high finesse even for axial beams because of diffraction. In addition, the extension of this approach to fluid samples is not described in the prior art.
Accordingly, what is needed in the art is a microcavity arrangement capable of sustaining the double resonance condition with high finesse for Raman scattering in fluids, thereby providing for the measurement of the concentration of one or more species of interests in a fluid sample.